1. Field of the Invention
The present invention relates to a control device, particularly to an energy-saving control device.
2. Description of the Related Art
Due to the fact that internal elements of most electric devices require direct-current (DC) voltages, a power supply is used to convert alternating-current (AC) voltages into the DC voltages such that the electric device operate well. The power supply is divided into a linear power supply and a switching power supply by circuit architecture. In order to obtain different power, the switching power supply is divided into Flyback, Forward, Full Bridge, Half-Bridge and Push-Pull power supply.
Take Flyback power supply for example. As shown in FIG. 1 and FIG. 2, a power converter circuit includes a transformer 10 having a primary winding PW and a secondary winding SW and an N-channel metal oxide semiconductor field effect transistor (NMOSFET) 12 connected to the primary winding PW. The secondary winding SW is connected with an IC 14. An input high voltage VH is applied to the primary winding PW. A voltage VG is periodically applied (TON) to the gate of the NMOSFET 12 to control the transfer of power from the primary winding PW to the secondary winding SW. When the NMOSFET 12 is turned on, the energy is stored in the transformer 10. As the NMOSFET 12 is turned off, the stored energy in the transformer 10 is discharged.
A reflected voltage VR is generated when the NMOSFET 12 is turned off. As a result the voltage VDS across the NMOSFET 12 equals the input high voltage VH plus the reflected voltage VR. While the NMOSFET 12 is turned off, a parasitic drain-to-source capacitor inherent in the NMOSFET 12 stores the energy from the voltage VD.
After a discharge period TDS the energy of the transformer 10 is fully discharge and the energy stored in the parasitic drain-to-source capacitor flows back to the input high voltage VH through the primary winding PW of the transformer 10.
The primary winding PW and the parasitic drain-to-source capacitor along with parasitic elements on the secondary winding create a resonant tank with a resonant frequency fR. While resonating, energy flows back and forth between the primary winding PW and the parasitic drain-to-source capacitor.
When the energy is discharged, an energy signal is generated at the drain. The energy signal has a high-frequency part and a low-frequency part. If the low-frequency part has a period larger than the high-frequency part and the IC 14 still operates, the operation of the IC 14 results in power loss.
To overcome the abovementioned problems, the present invention provides an energy-saving control device, so as to solve the afore-mentioned problems of the prior art.